Current transformer

ABSTRACT

A current transformer switchable at the primary side and operable in an electrical system at high voltage and current for detecting currents in respective current ranges is disclosed. The current transformer includes a secondary system defining a window passage therethrough and having an iron core. A plurality of primary windings surround the core and the windings include respective conductors having respective cross-sections stepped down in magnitude one with respect to the other in correspondence to respective ones of the current ranges. A metallic enclosure surrounds the secondary system in the manner of a hollow ring and constitutes a turn of the primary windings. An elongated conductor member in the form of a compact, short rod extends through the window passage for detecting currents in an additional current range greater than the first-mentioned current ranges.

United States Patent [191 Hermstein et al.

[ CURRENT TRANSFORMER [73] Assignee: Siemens Aktiengesellschaft, Munich,

Germany 22 Filed: Feb. 6, 1973 [21] App1.No.: 329,978

[30] Foreign Application Priority Data Feb. 7, 1972 Germany 7204896 [52]US. Cl 336/82, 336/96, 336/174, 336/175, 336/195 [51] Int. Cl. Holf40/06 [58] Field of Search 336/173, 174, 175, 82, 336/96, 195

[56] References Cited UNITED STATES PATENTS 1,953,779 4/1934 Sclater etal 336/174 X 1,986,884 l/1935 Rassler 336/82 3,287,679 11/1966 Jensen etal. 336/174 3,562,457 2/1971 Prek 336/174 X FOREIGN PATENTS ORAPPLlCATlQNS 763,802 5/1934 France 336/173 1,231,346 12/1966 Germany336/175 [11] 3,835,429 1 Sept. 10, 1974 Primary ExaminerThomas J. KozmaAttorney, Agent, or Firm-Kenyon & Kenyon Reilly Carr & Chapin [5 7]ABSTRACT A current transformer switchable at the primary side andoperable in an electrical system at high voltage and current fordetecting currents in respective current ranges is disclosed. Thecurrent transformer includes a secondary system defining a windowpassage therethrough and having an iron core. A plurality of primarywindings surround the core and the windings include respectiveconductors having respective crosssections stepped down in magnitude onewith respect to the other in correspondence to respective ones of thecurrent ranges. A metallic enclosure surrounds the secondary system inthe manner of a hollow ring and constitutes a turn of the primarywindings. An elongated conductor member in the form of a compact, shortrod extends through the window passage for detecting currents in anadditional current range greater than the first-mentioned currentranges.

18 Claims, 8 Drawing Figures PAIENIwsEP 1 om:

, sum 1 or 3 Fig. 2 I g Pmmmsim I 3-.835,429

SHEET 3 BF 3 BACKGROUND OF THE INVENTION The invention relates to acurrent transformer for high voltage and current. The currenttransformer is switchable on the primary side and has several windingssurrounding the iron core as well as a conductor crosssection decreasingaccording to the current ranges. For a further current range which isthe highest among the current ranges of the windings, an additional,elongated conductor passes through the window of the secondary system inthe form of a solid, short rod.

Deutsche Auslegungsschrift 1,231,346 discloses a current transformerwherein the windings consist of one or several conductors which are ledaround the secondary system. The capability of the known transformer towithstand current surges is therefore limited in the ranges in which thewindings are connected by the dynamic strength of the primary windings.

SUMMARY OF THE INVENTION Accordingly, it is an object of the inventionto provide a current transformer switchable on the primary side whichnot only has a high dynamic strength with respect to the highest currentrange, for example, as when the rod or the rod and one turn areconnected, but also provides a good capability to withstand surgecurrents if primary windings for measuring currents in lowercurrent-measuring ranges are connected.

The current transformer of the invention is switchable at the primaryside and operable in an electrical system at high voltage and currentfor detecting currents in respective current ranges. According to afeature of the invention, the current transformer includes a secondarysystem defining a window passage therethrough and having an iron core. Aplurality of primary windings surround the core and the windings includerespective conductors having respective cross-sections stepped down inmagnitude one with respect to the other in correspondence to respectiveones of the current ranges. A metallic enclosure surrounds the secondarysystem in the manner of a hollow ring and constitutes a turn of theprimary windings. An elongated conductor member in the form of acompact, short rod extends through the window passage for detectingcurrents in an additional current range greater than the first-mentionedcurrent ranges. According to another feature, the enclosure can be ahousing enclosing the secondary system and having a shape correspondingto a hollow ring.

Although it is already known from Deutsche Offenlegungsschrift 1,263,435to use the head housing as a turn in a high-voltage current transformerof head-type configuration which is switchable on the primary side, theknown high-voltage current transformer is suitable only for switching inthe ratio of l 2. The reason for this is that the known high-voltagecurrent transformer comprises no additional primary windings besides ashort, rod-shaped conductor for measuring the largest currents and thehousing as a further turn.

The above-mentioned enclosure has an end face at the window passage. Inthis end face, the enclosure has a ring-shaped opening in surroundingrelation to the window passage. The opening is closed with insulatingmaterial. A fluid insulating medium in the form of a gas or a liquid canfill the enclosure. An insulating plate covers the opening in afluidtight manner. The plate in turn has openings for passing throughand holding the compact rod as well as for accommodating the conductorsof the windings.

According to another feature of the invention, the insulating materiallocated inside the metallic enclosure or the metallic housing may alsoconsist of cast resin. In such a case the metallic enclosure may beformed by a conductive coating applied to the cast resin body. As willbe explained in more detail below, a metallic housing can be provided ifrequired, this being in addition to the metallic enclosure of a castresin insulation.

In the current transformer according to the invention, the windings orthe turns thereof can be arranged in different ways relative to thehousing. It is advantageous in the case of a winding with a single turnto have this turn formed by the housing, because of all windings, exceptfor the rod, the largest current flows through this winding if thecurrent transformer has been switched accordingly.

It may also be advantageous that one winding consists of at least oneturn disposed within the metallic enclosure or within the housing, andthat an additional winding consists of at least one turn placed withinthe enclosure or within the housing and a further turn formed by thehousing.

In the case of a metallic enclosure filled with cast resin, or of asimilarly filled housing, the turns located within the enclosure or thehousing can consist of rounded metal strips which are fitted to thecurvature of the enclosure or the housing.

It is particularly advantageous to accommodate the turns located withinthe housing in ribs of the housing. These ribs can then serve at thesame time to reinforce the housing.

It is also advantageous if the secondary system encapsulated within thecast resin is appreciably smaller than the housing and if the turns runin the space between the metallic enclosure and the housing, because theturns are then arranged so as to be free of field interference. By meansof a further casting about the first casting, the first cast resin bodyand the turns are then secured in the metallic housing in an insulatedmanner.

However, it is also possible and may in some cases be also particularlyadvantageous, to arrange the windings on the outside of the metallicenclosure or on the outside of the metallic housing; this affords theadvantage that no feedthrough terminals at all need be brought into theinner space of the housing. If required, this space may be filled with agaseous or liquid insulating medium. In this case a joint, for example,between a first and a second winding can be eliminated.

The conductors of the windings situated on the outside of the housing orthe metallic enclosure can be insulated against each other, against thehousing, and can be supported at the housing in different ways. Theconductors can, for example, have an insulating coating and/or may beconnected with the metallic enclosure or with the metallic housing bymeans of insulating pieces.

On the other hand, it is also possible to accommodate the conductors ofthe windings in tubing, which if the conductor is coated withinsulation, may consist of metal. In the absence of insulation on theconductors or in addition to such conductor insulation, the conductorscan be accommodated in insulating tubing. The tubing need to be only sothick that it can be readily bent. It is preferably made of such sizethat the tubing together with the solidrod substantially fills thewindow of the secondary system and is therefore braced; this enables thetransformer to withstand high surge currents. I it However, it is alsopossible to provide support bodies of insulating material in the windowof the secondary system; these support bodies serve to support thewinding conductors in the window, and also serve to achieve a highcapability to withstand surge currents if the conductors themselves donot completely fill the core window.

Although the invention is illustrated and described herein as a currenttransformer, it is nevertheless not intended to be limited to thedetails shown, since various modifications may be made therein withinthe scope and the range of the claims. The invention, however, togetherwith additional objects and advantages will be best understood from thefollowing description and in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a currenttransformer according to the invention. The transformer is here shown ashaving a head-type-configuration.

FIG. 2 illustrates an alternate embodiment of the current transformerof- FIG. 1 wherein only the .head portion is shown. In this embodimentthe windings extend outside of the head housing.

FIG. 3 is a further embodiment of the current transformer of theinvention wherein cast resin insulation is provided. The turns of thewindings are disposed within the insulation.

FIG. 4 is a section view of the head portion of the current transformershown in FIG. 3 taken transverse tothe longitudinal axis of the headportion.

FIG. 5 is also a section view of the head portion of the currenttransformer shown in FIG. 3 modified so as to illustrate the dispositionof winding turns disposed in ribs of the housing according to asubsidiary embodiment of the invention.

FIG. 6 illustrates still another embodiment of the invention wherein thewinding turns extend along the exterior of the housing of the headportion.

FIG. 7 is a section view of the head portion of the current transformerof FIG. 6 and is modified to illustrate how the winding turns can be runin tubing extending about the exterior of the housing according toanother subsidiary embodiment of the invention.

FIG. 8 illustrates a current transformer of the invention wherein theturns of the primary winding extend in a space between the housing andthe secondary system of the transformer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION plane of thedrawing. The head housing contains a secondary system 5 which consistsof several iron cores 6 and 7 with secondary windings 8 and 9. Thesecondary system 5 is embedded in a body of cast resin or surrounded byan oil-paper insulation-10 which extends into the insulating member 2.The secondary system 5 is accommodated in a metallic head housing 11having the shape of a hollow ring. The housing 11 has a circular opening12 at one of its end faces. The opening 12 is closed by an insulationplate 13 so as to be liquidtight.

The insulation plate 13 has a central cutout 14 through which a short,solid rod 15 is brought out as the face of the housing 11 by a supportmember 16. On that side, the rod 15 is connected directly with the powersystem. At its other end, the solid rod 15 is connected to a contact 17which can be connected by means of a switching strap 18 to the conductor'19 which represents the other connection of the current transformer tothe system.

In addition to the solid rod 15 as the primary conductor, a winding 20is disposed in the head housing 11 of the current transformer; thiswinding consists of onlya single turn and is brought with its one endfrom the outside into the housing 11 through a cutout 21 in theinsulation plate 13; this turn is connected at its other enddirectlywith the solid rod 15. The end of the primary winding 20 whichis not connected with the rod 15 is brought externally to a switchingcontact 22, which can I likewise be connected with the conductor 19 bymeans of the switching strap 18.

. In addition to the winding 20, the current trans- .former alsocomprises a further primary winding 23 which consists of a turnextending inside of the housing 11,, and of anadditional turn which isformed by the head housing 1.1. At the point 24, the first turn ismetallically connected to the housing l1.v Between the housing 11 and apoint 25 on the turn of the first primary winding 20, a conductor 26 isprovided for making a metallic connection. The start of the furtherprimary winding 23 is brought out through a cutout 27 in the insulatorplate. 13 to a contact 28. In the position of the switching strap 18shown, the contact 28 is connected to the conductor 19.

It will be seen from FIG. 1 that for the position of the switching strap18 shown, the simple nominal current can flow; the lowest current rangeis therefore connected. The current transformer then comprises a seriescircuit consisting of the additional primary winding 23 and the primarywinding 20 and the solid rod 15, and is therefore excited on the primaryside by four turns. If the switching strap 18 is switched to the contact22, the winding 20 is connected in series with the rod 15 and istherefore switched to the next higher current range. Twice the nominalcurrent can then be measured. If, however, the switching strap 18 isconnected to the contact 17, only the solid rod 15 is effective and thehighest current range is thereby switched in and four times the nominalcurrent can be measured.

In the head housing according to FIG. 2, only the secondary system 31with its corresponding insulation is accommodated inside the housing 30.Within the window of the secondary system 31 or expressed otherwise, inthe hollow ring-shaped housing 30, the solid rod 33 extendslongitudinally for the highest current range, as well as a first turn 34and a further turn 35 of a primary winding which comprises a total oftwo turns. The one end of this primary winding is connected to a contact36 of a switching arrangement, while the other end of the primarywinding with the turns 34 and 35 is connected with the metallic housing30. The metallic housing 30 constitutes one turn of a further primarywinding, which consists of only this one turn. The housing 30 istherefore connected on the one hand with a contact 38 of the switchingarrangement and, on the other hand, directly to the solid rod 33.

The housing 30 again has a circular opening 39, which is closed off bymeans of an insulation plate 40 in a liquid-tight manner. In theinsulation plate 40, cutouts for bringing out the winding conductors aswell as the solid rod are provided. The plate 40 therefore serves as asupport element for the primary conductor and contributes to increasethe dynamic strength. On the opposite side of the head housing 30 isprovided a support member 41, which is arranged within the window 32 andserves for supporting the primary conductors.

In the embodiment of FIG. 3 the secondary system is supported by agrounded core mounting 51 and is embedded in a cast resin body 52. Thecast resin body 52 is surrounded by a metallic housing 53 which is openat one end face. This opening can be used to place the casting resin inthe housing 53.

In the embodiment shown in FIG. 3 only the solid rod 55 is disposed inthe window 54; the other turns are embedded in the cast resin body 52.Thus, a primary winding consisting of one turn 56 is arranged inside thecast resin body 52 and brought with its one end to an external terminal57 which represents a contact of a switching arrangement (not shown).The other end of the turn 56 is brought to another external terminal 58which is connected with the solid rod 55. A further primary windingconsists of a turn 59 embedded in the cast resin body 52. The turn 59 isconnected to an external terminal 60 which serves as a contact of theswitching arrangement. The other end of the turn 59 is connected with ametallic housing 62 at a point 61. The housing 62 therefore formsanother turn of the further primary winding. The housing 62 is connectedvia a connecting line 63 to the end of the turn-56 which in turn isconnected to the external terminal 57. In this embodiment no insulationplate is therefore necessary and, accordingly, there is also no seal forthis plate.

In the embodiment of FIG. 4 a secondary system is shown embedded in acast resin body 71. A solid rod 73 is again situated in the window 72.Turns 74 and 75, which are comparable to the turns 56 and 59 accordingto FIG. 3, consist in the embodiment of FIG. 4 of rounded metal strips,which are fitted to the curvature of the housing 76 and can therefore bedisposed in the immediate vicinity of the wall of the housing.

FIG. 5 shows that the housing 80 is provided with ribs 81 and 82 inwhich the rounded metal strips of turns 83 and 84 are held. The ribs 81and 82 advantageously reinforce the mechanical strength of the headhousing 80. The area surrounding the window 85 of the housing 86 is alsoprovided with cutouts 87 and 88 for accommodating the windingconductors.

In the embodiment of FIG. 6 the secondary system 90 is again embedded ina cast resin body 91. The cast resin body 91 is surrounded on theoutside by a housing 92 which again has a circular opening 93 at its endface. In this embodiment, no turns of the primary winding are led withinthe cast resin body 91; instead, the turns run outside of the cast resinbody 91 and outside of the housing 92. Thus, a primary winding with itsonly turn 94 is first led from a contact 95 through the window 96 andoutside around the housing 92 to a joint 97 with the solid short rod 98.Likewise, within the window 96 is disposed the conductor of a turn 99 ofanother primary winding; this conductor extends from a contact 100 andis metallically connected to head housing 92 at the point 101 after oneturn. The head housing 92 forms a further turn of this primary windingby being connected, via a connecting conductor 102, with the end of theturn 94 that is connected to the contact 95; this junction is made atthe point 103.

Referring still to FIG. 6, the conductors of the primary winding areprovided with an insulating coating and are arranged so as to beinsulated by the insulator pieces 104 and 105 relative to the housing92. Insulators 106 and 107 are provided for securing the conductorswithin the window 96.

The insulators 104 to 107 become unnecessary if, as is'shown in FIG. 7,the conductor 110 of the one primary winding and the conductor 111 ofthe other primary winding are run in tubing 112 and 113; the conductors110 and 111 can be compared with the conductors of the turns 94 and 99of the embodiment according to FIG. 6. The tubes 112 and 113 are ofrelatively great stiffness, and their diameter can be chosen so that,together with the solid rod 115, they fill the window 114 to a greatextent; the primary conductors are thereby braced in the window withrespect to each other and this achieves a high capability to withstandcurrent surges.

The invention provides a switchable current transformer which isdistinguished by its capability to withstand current surges. The currenttransformer can be used in this connection not only for switching in theratio 1 2 4 but also for any other switching ratios if its primarywindings are configured accordingly.

FIG. 8 illustrates still another embodiment of the invention wherein ametal housing encloses a body of casting resin 121 containing thesecondary system. The secondary system enclosed by the casting resin issubstantially smaller than the housing 120 so that the housing 120 andthe body of casting resin 121 conjointly define a space 124 wherein theturns are run. These turns are then not subjected to field disturbances.An additional casting within the housing 120 can be provided to insurethat the turns and the cast resin body 121 are insulated in fixedposition inside housing 120.

What is claimed is:

1. A current transformer switchable at the primary side and operable inan electrical system at high voltage and current for detecting currentsin respective current ranges comprising a secondary system defining awindow passage therethrough and having an iron core; a plurality ofprimary windings surrounding said core, said windings includingrespective conductors having respective cross-sections stepped down inmagnitude one with respect to the other in correspondence to respectiveones of the current ranges; a metallic enclosure surrounding saidsecondary system in the manner of a hollow ring and constituting a turnof said primary windings, said enclosure having a wall portion extend-ring throughsaid window passage so as to cause said enclosure to beelectromagnetically linked with-said secondary system, and an elongatedconductor member in the form of a compact, short rod extending throughsaid window passage for detecting currents in an additional currentrange greater than the first-mentioned currentranges.

2. The current transformer of claim 1, said enclosure being a housingenclosing said secondary system and having a shape corresponding to ahollow ring.

3. The current transformer'ofclaini lgsaid enclosure having an end faceat said window passage; said enclosure having a ring-shapedopeninginsaidend face in insulating body of cast resindisposediinsidesaidienclosure, said'secondary system being imbedded insaid body of cast resin.

'6, The current transformer of claim said metal enofcasting resin." I

' closure being a Fm f to Said body sure surroundirig said secondarysystem in the manner of'a hollow ring and constituting a turnof saidprimary windings; an elongated conductor member in the form. n of acompa ct, short rod extending through-said window passage fordetectingcurrents inan additional current 21f jrange'greatef'than thefirst-mentioned ciirreritrarigsi said, enclosure having an end face atsaid'window' as-1 sage, said enclosure havinga ring-shaped opening insaid end face in surrounding relation to said window passage; and, aninsulating body of cast'resin disposed 7. A current transformerswitchable at the primary 1 side andoperable inan electrical system athigh voltage and current for detecting currents in respective current:-

ranges comprising a secondary system defining al -win I dow passagetherethrough and having an ironffeo're; a plurality of primary windingssurroundin siaidfcote,

said windings including respective co respective cross-sections steppedo n mmagnifude one with respect totheotheri ondence'torespective onesofthe curren'tkranges; a' metallic enclosure surroundin s'aidsec'ondary'syst ern in the manner constituting a turn of said primarywindii ig s; 'an elongatedconductor member in the form of ia compact,short rod extending through said window yi I passage forgdetectingcurrents in an additional current 'r'ap et greater than thefirst-mentioned current,rahges1;'-

' 'enclosurehaving an end face at said window pas sage'fsaidenclosurehaving a ring-shaped opening in of a hollow'ringa' saidend facein-surrounding relation to said window passage; and, an insulating bodyof cast resin disposed inside said enclosure, saidsecondarysystem beingimbedded in said body of castresin, said primary windings havingrespectiv'e'flturns disposed in said body of cast resin,saidturnsbeing'respective rounded metal strips, each of said metalstrips'having a curvature corresponding to thecurvature of saidenclosure.

'8; Thejc'urrent transformer of claim 7, said enclosure hayihgribs,saiditurns being accommodated in said I ribs. I i 9. A- currenttransformer switchable at the primary in side and operable in anelectrical system at high voltage and'current for detecting currents inrespective current ranges comprising a secondary system defining awindow passage therethrough and having an iron core; a plurality ofprimary windings surrounding said core, said windings includingrespective conductors having respective cross-sections stepped down inmagnitude one with respect to the other in correspondence to respectiveones of the current ranges; a metallic encloors having sure surroundingsaid secondary systemin the manner of a hollow ring and constitutingaturn'of said primary windings; an elongated conductor member in theform of a compact, short rod extending through said window passage fordetecting'currents in an additionalcurrent havingrespective turnsdisposed in said space 10. Acurrent transformer switchable at.

ranges comprising a secondary enclosure.

- llfThe current transformer of claim 10 comprising insulating piecesfor mounting the conductors of s'ai'd turns on said exterior of; saidenclosure. H

12. The current transformer of claim 10, comprisingtubes extending alongthe exterior of said enclosure, the conductors of said turns being runin said tubes.

13. The current transformer of claim 12, said tubes being made of metal.

14. The current transformer of claim 12, said tubes being made ofinsulating material.

15. The current transformer of claim 12, a portion of each of said tubesextending through said window passage, said portions of said tubes andsaid elongated conductor member-substantially filling said windowpassage.

16. The current transformer of claim 10, portions of the conductors ofsaid turns extending through said window passage, the currenttransformer comprising support bodies of insulating material disposed insaid window passage for supporting said portions of said conductors andsaid elongated conductor member therein.

17. The current transformer of claim 1, one of said windings of saidplurality of windings consisting of a singleturn, said single turn beingformed by said metallic enclosure.

. e p ufi side and operable in an electrical system high voltage andcurrent for detecting curren re's'pectivecurrent e' n' finin awn dowpassage therethroughandihaving; an-ironcore;a;;, I plurality of primarynqi gss n anqmg said' core," said windings includingirespe ctiveconductorshavin gfl; respective cross-sections steppedfdown in magnitudeone with 'respe'ct' tothe other in correspondence to re-.

10 posed within said metal enclosure and one further turn, said furtherturn being formed by said metallic enclosure.

1. A current transformer switchable at the primary side and operable inan electrical system at high voltage and current for detecting currentsin respective current ranges comprising a secondary system defining awindow passage therethrough and having an iron core; a plurality ofprimary windings surrounding said core, said windings includingrespective conductors having respective cross-sections stepped down inmagnitude one with respect to the other in correspondence to respectiveones of the current ranges; a metallic enclosure surrounding saidsecondary system in the manner of a hollow ring and constituting a turnof said primary windings, said enclosure having a wall portion extendingthrough said window passage so as to cause said enclosure to beelectromagnetically linked with said secondary system, and an elongatedconductor member in the form of a compact, short rod extending throughsaid window passage for detecting currents in an additional currentrange greater than the first-mentioned current ranges.
 2. The currenttransformer of claim 1, said enclosure being a housing enclosing saidsecondary system and having a shape corresponding to a hollow ring. 3.The current transformer of claim 1, said enclosure having an end face atsaid window passage; said enclosure having a ring-shaped opening in saidend face in surrounding relation to said window passage.
 4. The currenttransformer of claim 3, comprising a fluid insulating medium fillingsaid enclosure; and an insulating plate for covering said openingfluid-tight, said plate in turn having openings for passing through andholding said compact rod as well as for accommodating said conductors ofsaid windings.
 5. The current transformer of claim 3, comprising aninsulating body of cast resin disposed inside said enclosure, saidsecondary system being imbedded in said body of cast resin.
 6. Thecurrent transformer of claim 5, said metal enclosure being a conductivecoating applied to said body of casting resin.
 7. A current transformerswitchable at the primary side and operable in an electrical system athigh voltage and current for detecting currents in respective currentranges comprising a secondary system defining a window passagetherethrough and having an iron core; a plurality of primary windingssurrounding said core, said windings including respective conductorshaving respective cross-sections stepped down in magnitude one withrespect to the other in correspondence to respective ones of the currentranges; a metallic enclosure surrounding said secondary system in themanner of a hollow ring and constituting a turn of said primarywindings; an elongated conductor member in the form of a compact, shortrod extending through said window passage for detecting currents in anadditional current range greater than the first-mentioned curRentranges; said enclosure having an end face at said window passage, saidenclosure having a ring-shaped opening in said end face in surroundingrelation to said window passage; and, an insulating body of cast resindisposed inside said enclosure, said secondary system being imbedded insaid body of cast resin, said primary windings having respective turnsdisposed in said body of cast resin, said turns being respective roundedmetal strips, each of said metal strips having a curvature correspondingto the curvature of said enclosure.
 8. The current transformer of claim7, said enclosure having ribs, said turns being accommodated in saidribs.
 9. A current transformer switchable at the primary side andoperable in an electrical system at high voltage and current fordetecting currents in respective current ranges comprising a secondarysystem defining a window passage therethrough and having an iron core; aplurality of primary windings surrounding said core, said windingsincluding respective conductors having respective cross-sections steppeddown in magnitude one with respect to the other in correspondence torespective ones of the current ranges; a metallic enclosure surroundingsaid secondary system in the manner of a hollow ring and constituting aturn of said primary windings; an elongated conductor member in the formof a compact, short rod extending through said window passage fordetecting currents in an additional current range greater than thefirst-mentioned current ranges; said enclosure having an end face atsaid window passage, said enclosure having a ring-shaped opening in saidend face in surrounding relation to said window passage; an insulatingbody of cast resin disposed inside said enclosure, said secondary systembeing imbedded in said body of cast resin; and, a housing surroundingsaid enclosure and said cast resin body containing said secondarysystem, said enclosure and said housing defining a space therebetween,said primary windings having respective turns disposed in said space.10. A current transformer switchable at the primary side and operable inan electrical system at high voltage and current for detecting currentsin respective current ranges comprising a secondary system defining awindow passage therethrough and having an iron core; a plurality ofprimary windings surrounding said core, said windings includingrespective conductors having respective cross-sections stepped down inmagnitude one with respect to the other in correspondence to respectiveones of the current ranges; a metallic enclosure surrounding saidsecondary system in the manner of a hollow ring and constituting a turnof said primary windings; an elongated conductor member in the form of acompact, short rod extending through said window passage for detectingcurrents in an additional current range greater than the first-mentionedcurrent ranges; said enclosure having an end face at said windowpassage, said enclosure having a ring-shaped opening in said end face insurrounding relation to said window passage; and, an insulating body ofcast resin disposed inside said enclosure, said secondary system beingimbedded in said body of cast resin, said primary windings havingrespective turns mounted on the exterior of said enclosure.
 11. Thecurrent transformer of claim 10 comprising insulating pieces formounting the conductors of said turns on said exterior of saidenclosure.
 12. The current transformer of claim 10, comprising tubesextending along the exterior of said enclosure, the conductors of saidturns being run in said tubes.
 13. The current transformer of claim 12,said tubes being made of metal.
 14. The current transformer of claim 12,said tubes being made of insulating material.
 15. The currenttransformer of claim 12, a portion of each of said tubes extendingthrough said window passage, said portions of said tubes and saidelongated conductor member substantially filling said window passage.16. The current transformer of claim 10, portiOns of the conductors ofsaid turns extending through said window passage, the currenttransformer comprising support bodies of insulating material disposed insaid window passage for supporting said portions of said conductors andsaid elongated conductor member therein.
 17. The current transformer ofclaim 1, one of said windings of said plurality of windings consistingof a single turn, said single turn being formed by said metallicenclosure.
 18. The current transformer of claim 1, said plurality ofwindings being at least two in number, one of said two windingsconsisting of at least one turn being run within said metallicenclosure, and said other one of said two windings consisting of atleast one turn disposed within said metal enclosure and one furtherturn, said further turn being formed by said metallic enclosure.